Ring-wing aircraft

ABSTRACT

This invention relates to ring-wing aircraft and is suited particularly, although not exclusively, to use in micro-unmanned air vehicles (UAV&#39;s) with ring-wings.  
     An aircraft ( 10 ) according to the invention comprises a ring-wing ( 11 ) defining a duct ( 16 ) with a longitudinally-extending central axis ( 31 ), propulsion means ( 15 ) located within the duct and moveable aerofoils ( 13, 18 ) for controlling the aircraft in flight, the ring-wing being truncated obliquely at one end, that end being the rear ( 11   b ) when in horizontal flight, to form a ring-wing with opposed sides of unequal length. This arrangement produces center of mass offset from the central axis of the ring-wing, the pendulum effect will ensure that the aircraft will roll so that its center of mass will always be at the lowest height possible when the aircraft is airborne. Therefore the aircraft has a preferred orientation, and the control surfaces can be oriented with respect to this preferred orientation. In addition, the oblique truncation at the rear keeps the center of mass towards the front of the aircraft thereby giving improved stability in all three axes.

[0001] This invention relates to ring-wing aircraft and is suitedparticularly, although not exclusively, to use in micro-unmanned airvehicles (UAV's) incorporating ring-wings. However, it will beappreciated that the invention is equally well suited to use inring-wing aircraft of any size, whether they be manned or unmanned.

[0002] Unmanned air vehicles have found a number of applications, wherethey have been used to carry a wide variety of payloads. In the absenceof a pilot, the aircraft may be scaled down, typically to a size of 150mm or less. The reduction in size and the absence of extra load in theform of a pilot fit in well with the constant drive to produce lighteraircraft because of their inherent efficiency advantages.

[0003] In terms of flexibility it is advantageous for UAV's to be ableboth to hover and to fly horizontally at high speed. This criterion hasled to the development of ring-wing aircraft, i.e. an aircraft having apropeller or other propulsion system mounted within a duct defined by aring-shaped wing of substantially circular cross-section. A ring-wingaircraft is disclosed in U.S. Pat. No. 5,295,643 to Hughes MissileSystems Company, although the scale of the aircraft is not disclosed.The Hughes aircraft has a ‘toroidal’ ring-wing that defines a duct(although the ring-wing is generally flat in cross-section so that itcan be considered as essentially an annular cylinder). A propeller,stators and vanes are located within the duct. In the Hughes aircraft,the stators serve to straighten the swirling air produced by therotating propeller and hence help to balance the torque applied to theaircraft by the rotating propeller. The vanes are used as controlsurfaces for controlling the pitch and yaw of the aircraft whenairborne.

[0004] The use of a ring-wing has benefits that include reducing thenoise signature of the propeller and providing a protective housing forthe propeller that guards against both damage to the propeller anddamage to anything or anyone that may otherwise come into contact withthe propeller. However, the main advantage of the ring-wing design isthat it can be used both for hovering, by orienting the propellerhorizontally, and for horizontal flight by orienting the propellervertically. In the Hughes aircraft, the transition from vertical tohorizontal flight is effected by use of the vanes.

[0005] However, the Hughes Patent does not discuss any aspects of rollpertinent to their aircraft, nor does it contain any detailed discussionon any aspects of yaw or pitch. Clearly these are important aspects onthe design of a ring-wing UAV as these aircraft must be controlledeither autonomously or by remote control, and so control and stabilityof the aircraft must be enhanced, say, over a manned vehicle where thepilot will have direct feedback from his sense of balance as to theattitude of the aircraft. Where autopilots are used, natural stabilityof the UAV is highly important as it reduces the complexity, size and,critically, the mass of the autopilot system to be installed on theaircraft.

[0006] Natural stability in the pitch and yaw planes is achieved byplacing the centre of mass of the aircraft ahead of the aerodynamicneutral points in the pitch and yaw planes. The shape of the ring-wingmeans that the aerodynamic pressures acting on all portions of thering-wing have a line of action that inherently passes through thecentre of the ring. The symmetry of the continuous ring-wing ensuresthat these pressures give rise to no contribution to rolling moment. Itwill be appreciated that the ring-wing need not be continuous: the ringmay be divided into two or more sections and there will still be norolling moment contribution from the aerodynamic pressures.

[0007] However, the inherent neutral symmetry of ring-wing aircraft canbe considered problematic. This is because any disturbance about theroll axis leaves the aircraft in an attitude without any incrementalaerodynamic rolling moment to restore it to its undisturbedcondition—there is no tendency for the aircraft to right any inducedroll. This tendency freely to roll can lead to a requirement for aflight control system to provide a means of controlling the vehicle,thereby adding complexity, and quite often mass, to the aircraft.

[0008] An object of the invention is to overcome the disadvantages ofprevious designs of ring-wing UAV's described hereinabove.

[0009] From a first aspect, the present invention resides in an aircraftcomprising a ring-wing defining a duct with a longitudinally extendingcentral axis, propulsion means located within the duct and moveableaerofoils for controlling the aircraft in flight, the ring-wing beingtruncated obliquely at one end, that end being the rear when inhorizontal flight, to form a ring-wing with opposed sides of unequallength.

[0010] It should be noted that the term ring-wing is intended toencompass rings of substantially circular cross-section, both continuousand interrupted, i.e. where segments of the ring have been omitted. Thisis best done to leave a wing symmetrical about its central axis.

[0011] It will be appreciated that by truncating the ring-wing obliquelyat its rear produces and aircraft with a centre of mass offset from thecentral axis of the ring-wing. The pendulum effect will ensure that thecentre of mass will always be at the lowest height possible when theaircraft is airborne. Hence, should the aircraft roll, the rollingmoment due to the increased height of the centre of mass will produce arolling motion of the aircraft so that the centre of mass returns to thelowest height possible. It will be appreciated that the further thecentre of mass is offset from the central axis of the ring-wing, thegreater the tendency for the aircraft to roll to correct any disturbanceabout its roll axis. Therefore the aircraft has a preferred orientation,and the control surfaces can be oriented with respect to this preferredorientation. Roll stability is thus achieved through the restoringmoments provided by the offset centre of gravity rather than throughaerodynamic restoring moments.

[0012] In addition, truncating the ring-wing at its rear rather that itsfront is advantageous as it produces a centre of mass ahead of theaerodynamic neutral points in the pitch and yaw planes when the aircraftis engaged in high-speed forward flight. It should be appreciated thatthe applicant has realised that a simple modification of the basicring-wing shape is highly beneficial because in addition to offsettingthe centre of mass from the central axis of the ring-wing to provideself-righting roll control, it also moves the centre of mass towards thefront of the aircraft, i.e. the simple modification provides muchimproved stability in all three axes.

[0013] Optionally, the ring-wing is truncated by a planar slice throughthe ring-wing. It is preferred that the planar slice makes an angle ofbetween 25° and 65° to the longitudinally extending central axis. It isfurther preferred for the angle to be between 25° and 45° and yetfurther preferred for the angle to be substantially equal to 45°.Optionally, the ring-wing is truncated by a curved slice, the angle madewith the ring-wing being relatively shallow at the longer side of thering-wing and relatively steep at the shorter side of the ring-wing.This arrangement conveniently produces a greater shift in the centre ofmass to the shallow end.

[0014] The payload and other components of the aircraft that do not needto be located on the central axis of the ring-wing (e.g. remove controlreceiver, batteries or fuel tank, motor speed controller, etc.)constitute the substantial majority of the overall mass of the aircraftand, thus, should be located offset from the central axis of thering-wing. In order to maximise the pendulum effect, it is advantageousto house components and/or any payload in a compartment that is externalto the longer side of the ring-wing. Alternatively, or in addition,components and/or any payload may be housed in a compartment providedwithin the longer side of the ring-wing. Advantageously, the compartmentmay be provided at one end of the ring-wing, that end being the frontwhen in horizontal flight, as this configuration assists stability inall three axes.

[0015] The aircraft may have a pitch-controlling aerofoil componentprovided offset from the central axis of the ring-wing. Optionally, itmay be positioned to reside substantially flat with the longer side ofthe ring-wing when not deployed. Advantageously, it may be provided asan element of the longer side of the ring-wing.

[0016] Conveniently, the aircraft may further comprise two orthogonalvanes wherein one vane provides aerodynamic lift in the same directionas the pitch-controlling aerofoil. These vanes may primarily control theroll and yaw attitude of the aircraft, and they may operate inconjunction with the pitch-controlling aerofoil in certain manoeuvres,to effect a turn for example.

[0017] Optionally, any aircraft defined hereinabove is unmanned. Thediameter of the ring-wing may be less than 50 cm, although a diameter ofless than 25 cm is currently preferred.

[0018] The invention will now be described, by way of example only, byreference to the accompanying drawings in which:

[0019]FIG. 1 is a perspective view of an aircraft according to a firstembodiment of the present invention;

[0020]FIG. 2 is a plan view of the aircraft of FIG. 1;

[0021]FIG. 3 is a front view of the aircraft of FIG. 1;

[0022]FIG. 4 is a section along line IV-IV of FIG. 2;

[0023]FIG. 5 is a perspective view of an aircraft according to a secondembodiment of the present invention; and

[0024]FIG. 6 is a section along line VI-VI of FIG. 5.

[0025] As can be seen from FIGS. 1 to 3, an aircraft 10 according to afirst embodiment of the invention comprises a ring-wing 11 with acompartment 12 and a rear elevator 13. The compartment 12 is locatedbelow a forward section 11 a of the ring-wing 11 so that it extendsbeyond the front of the ring-wing 11 . The size of the aircraft can begauged from the fact that the ring-wing 11 has a diameter of 12 cm. Thering-wing 11 is obliquely truncated at an angle α of 45° to thering-wing's longitudinally-extending central axis 31 to define theforward section 11 a and a tail section 11 b comprising a part ring. Thering-wing 11 is truncated in planar fashion so that the tail section 11b tapers uniformly to meet the elevator 13 at the rear of the aircraft.The centreline of the tail section 11 b is coincident with thecentreline of the elevator 13.

[0026] Turning now to the front of the aircraft, a forward fairing 14 aprotrudes from the centre of a void 16 defined by the ring-wing 11. Theforward fairing 14 a is located directly in front of a four-bladedpropeller 15 which resides just inside the void 16 and rotates about thelongitudinal axis 17 of the ring-wing 11. Behind the propeller 15, arear fairing 14 b extends along the longitudinal axis to terminate abovethe tail section 11 b of the ring-wing 11. Four vanes 18 are locatedwithin the void 16 at the rear of the forward section 11 a and arearranged symmetrically around the longitudinal axis 17 of the ring-wing11 as a pair of vertical vanes 18 a and a pair of horizontal vanes 18 b.The vanes 18 protrude slightly beyond the extent of the top of theforward section 11 a of the ring-wing 11, i.e. they overlap slightlywith the tail section 11 b.

[0027] A compartment 12 is located underneath the ring-wing 11 so thatit spans the length of the forward section 11 a and protrudes forwardsof the leading edge 11 c of the ring-wing 11. The compartment 12 isprofiled to have an aerodynamic shape, i.e. its outer surfacesconstitute a further fairing.

[0028] A pair of elongate supports 19 span the void 16 at the rear ofthe forward section 11 a of the ring-wing 11, one horizontal and onevertical, and support the fairings 14 a, 14 b and the vanes 18.

[0029]FIG. 4 is a vertical section of the aircraft 10 and shows theaircraft in more detail. As can be seen, the compartment 12 is hollowand houses several components of the aircraft 10. Specifically, theseare a pair of high-performance batteries 20 to power the plane, a radioreceiver 21 to receive the flight control instructions transmitted by ahand-held transmitter used by an operator and an electronic motor speedcontroller 22. In addition, a payload 23 is also shown. To ensure anoptimal aerodynamic profile of the compartment 12, the components 20,21, 22 and payload 23 are arranged one in front of the other, therebyensuring that the compartment 12 is shallow and narrow. The aerodynamicprofile of the compartment 12 is clearly shown in FIG. 4.

[0030] Furthermore, FIG. 4 also shows the ring-wing 11 to have acharacteristic cross-sectional profile of aerofoils, thereby maximisingthe lift generated by the ring-wing 11. The aerodynamic profile of thering-wing 11 is uniform around its circumference. This typical aerofoilprofile is also used for the elevator 13 and for the vanes 18.

[0031] The propeller 15 is powered by a motor 24 housed in a cavityprovided in the rear fairing 14 b behind the propeller 15, the propeller15 being mounted on a central shaft of the motor 24. The body of themotor 24 forms part of the sides of the rear fairing 14 b. Recessing themotor 24 within the rear fairing 14 b ensures optimal aerodynamics ofthe aircraft 10. The supports 19 pass through the rear fairing 14 bbehind the motor 24 in through-holes sized to ensure a snug fit andterminate snugly within holes provided in the ring-wing 11. The supports19 are bonded securely within the holes provided in the ring-wing 11.

[0032] The vanes 18 are provided with through-holes extending across thewidth of their forward end, the through-holes being sized to provide asliding fit within the supports 19 which pass through the through-holes.Accordingly, the vanes 18 pivot about the supports 19 to provide controlsurfaces during flight. Each vane 18 is moved by an associated servomotor 25 a linked to its vane 18 by a crank arm and linkage 26 a. Theservo motors 25 a are housed within recesses 27 a of the ring-wing 11fitted with aerodynamic covers 28. Similarly, the elevator 13 is alsodeployed by a servo motor 25 b linked by a crank arm and linkage 26 b,the servo motor 25 b being housed within a covered recess 27 b of thetail section 11 b. All servo motors 25 a, 25 b draw their power from thebatteries 20, as does the propeller motor 24. Wires (not shown)distribute electricity and are routed within recesses or hollows.

[0033] The forward fairing 14 a is hollow, which is advantageous as theinternal space provides room for further payloads or, when not used as astore, reduces the overall mass of the aircraft 10.

[0034] This crucial aspect of the overall mass of the aircraft 10 drivesthe choice of materials of the various components of the aircraft 10.Low density materials are generally preferred. Specifically, styrofoamis used for the ring-wing 11, the compartment 12, the vanes 18, elevator13 and fairings 14 a, 14 b. Carbon-fibre composite tubes are used forthe supports 19 and also for the crank arms and linkages 26 a, 26 b incombination with spring-steel wires. Compact, low-mass servo motors 25a, 25 b and motor 24 are employed.

[0035] The careful choice of materials, component shape and positioningdescribed hereinabove results in a centre of mass 29 of the aircraft 10that is both below the central axis 17 of the ring-wing 11 and close tothe leading edge 11 c of the ring-wing 11. Therefore, the aircraft 10has considerable stability in the pitch and yaw planes and, in additionto stability about its roll axis, it is self-righting about its rollaxis.

[0036] Aspects relating to the flight of the aircraft 10 will now bedescribed. The aircraft 10 is launched using a conventional model glidertechnique, namely a long length of elastic material that launches theaircraft 10 into horizontal flight, the propeller 15 already running.

[0037] In level horizontal flight, the vanes 18 are not positionedexactly horizontally and vertically, but are slightly offset to accountfor the torque produced by the rotating propeller 15 which wouldotherwise produce a tendency for the aircraft 10 to roll. Control of theaircraft 10 in flight is accomplished using the elevator 13 for pitchcontrol and the vanes 18 for roll-control. Yaw control has been foundnot to be necessary—turns are accomplished by using a combination ofpitch and roll control in a conventional manner.

[0038] When hovering, the propeller 15 is oriented horizontally with theelevator 13 being lowermost. For this mode the elevator 13 controlspitch and the vanes 18 control both roll and yaw. Rather than beingoriented exactly upright, the aircraft 10 is tilted slightly in order tomaintain the centre of mass 29 in line with the thrust axis. For finecontrol of the thrust axis, the propeller 15 may have a conventionalhelicopter-like pitch/collective mechanism so that the angle of thepropeller blades 15 changes as they rotate from the approaching sectorto the retreating sector. Alternatively, this fine control could beaccomplished by mounting the motor 24 on gimbals thereby allowing thewhole axis of rotation of the propeller 15 to be tilted. The aircraft 10can be moved around at low speed in the hover mode by slight alterationof the tilt of the aircraft 10.

[0039] Transition between horizontal flight and hover is accomplishedusing a conventional pitch-up manoeuvre initiated using the elevator 13or vanes 18 combined with throttling of the motor 24. Transition fromhover to horizontal flight is accomplished in an inverse manner.

[0040] When the aircraft 10 rolls for any reason, for example ifdisturbed by a gust of wind or if rolled during a turning manoeuvre, therolling moment due to the increase in height gained by the centre ofmass 29 will produce a tendency to roll in the opposite sense so thatthe centre of mass 29 may return to its lowest height. Hence, whendisturbed by a gust of wind, the aircraft 10 will correct the inducedroll straight away without the need to use the vanes 18 or the elevator13. In the case of a turning manoeuvre, the aircraft 10 will self-rightto its stable in-flight orientation as soon as the vanes 18 and theelevator 13 are no longer deployed.

[0041] A second embodiment of the invention is shown in FIGS. 5 and 6.This embodiment broadly corresponds to the first embodiment, and so likereference numerals are used for like parts.

[0042] This alternative embodiment differs firstly in the design of thetail section 11 b of the ring-wing 11 and the elevator 13. The ring-wing11 is truncated obliquely along a curve, so that it cuts the top of thering-wing 11 at a relatively sharp angle and then becomes progressivelyshallower to meet a ‘V’-shaped beaver-tail section 11 b. A moveableelevator 13 is recessed into each side of the ‘V’ and is hinged at itsforward edge. These elevators 13 work in tandem to control the pitch ofthe aircraft 10 or in combination to control the roll of the aircraft10.

[0043] This alternative embodiment also differs in that several of thecomponents of the aircraft 10 are housed in a compartment located withinthe lower front portion of the ring-wing 11. Although the compartment isnot shown explicitly, its position is shown generally by the referencenumber 30 in FIG. 5.

[0044] The person skilled in the art will appreciate that modificationscan be made to the embodiments described hereinabove without departingfrom the scope of the invention.

[0045] For example, rather than using the vanes 18 to compensate for thetorque produced by the rotating propeller 15, a contra-rotatingpropeller could be employed. This also has the benefits of increasingthe thrust produced, provided the correct optimisation of the propellerblades is undertaken.

[0046] As an alternative to using styrofoam for parts of the aircraft10, they could be made from silica aero-gels which, advantageously, havevery low densities but retain high structural strength. Compositestructures including lightweight honeycomb fillers contained within athin shell could also be used. Where the invention is embodied as alarge aircraft, conventional aircraft materials could be used, forexample the ring-wing 11 could have a conventional structure of spars,struts and ribs. Whilst a profile that remains uniform around thering-wing 11 is used in the above embodiment, the profile could varyaround the ring-wing 11. For example, to gain extra lift, an exaggeratedcamber may be used on the top and bottom sections of the ring-wing 11,the camber being in the same sense for both parts. Clearly, the profileof the ring-wing 11 must vary between the top and bottom sections forthe camber to be in the same sense.

[0047] Furthermore, rather than operating the aircraft 10 by remotecontrol, the aircraft may be autonomous, employing an auto-pilot forexample.

1. An aircraft comprising a ring-wing of substantially circularcross-section defining a duct with a longitudinally-extending centralaxis, a propulsion means located within the duct and moveable aerofoilsfor controlling the aircraft in flight, the ring-wing being truncatedobliquely at one end, that end being the rear when in horizontal flight,to form a ring-wing with opposed sides of unequal length.
 2. An aircraftaccording to claim 1, wherein the ring-wing is truncated by a planarslice through the ring-wing.
 3. An aircraft according to claim 1,wherein the ring-wing is truncated by a curved slice, the angle madewith the ring-wing being relatively shallow at the longer side of thering-wing and relatively steep at the shorter side of the ring-wing. 4.An aircraft according to any preceding claim, wherein components and/orany payload are housed in a compartment that is external to the longerside of the ring-wing.
 5. An aircraft according to any preceding claim,wherein components and/or any payload are housed in a compartmentprovided within the longer side of the ring-wing.
 6. An aircraftaccording to claim 4 or claim 5, wherein the compartment is provided atan end of the ring-wing, that end being the front when in horizontalflight.
 7. An aircraft according to any preceding claim, wherein thepitch controlling aerofoil is positioned to reside substantially flatwith the lower side of the ring-wing when not deployed.
 8. An aircraftaccording to claim 7, wherein the pitch-controlling aerofoil is providedas an element of the longer side of the ring-wing.
 9. An aircraftaccording to claim 7 or claim 8, further comprising two orthogonalvanes, wherein one vane provides aerodynamic lift in the same directionas the pitch-controlling aerofoil.
 10. An aircraft according to anypreceding claim which is unmanned.